Although the expression of most genes is tightly regulated, some genes are expressed at roughly constant rates (i. e., constitutively). Which of the following genes would you predict to be constitutively expressed?
A. genes involved in the biosynthesis of tryptophan (an amino acid)
B. genes involved in the degradation of tryptophan
C. genes involved in the degradation of arabinose, a sugar
D. genes that that code for ribosomal RNAs
E. genes involved in the transport of maltose, a sugar, into the cell